For the uses of photopolymers for the production of holographic media, the crucial role is played by the refractive index constant Δn produced in the photopolymer by the holographic exposure. In holographic exposure, the interference field of signal light beam and reference light beam (that of two planar waves in the simplest case) is mapped into a refractive index grating by the local photopolymerization of, for example, high-refractive acrylates at loci of high intensity in the interference field. It is the refractive index grating in the photopolymer which is the hologram and which contains all the information in the signal light beam. By illuminating the hologram with only the reference light beam, the signal can then be reconstructed. The strength of the signal thus reconstructed relative to the strength of the incident reference light is called the diffraction efficiency, DE in what follows.
High-refractive acrylates are capable of producing refractive index gratings with high amplitude between regions with low refractive index and regions with high refractive index, and hence of enabling holograms with high DE and high Δn in the photopolymer. It should be noted here that the grating thickness and hence the DE depends on the product of Δn and the photopolymer layer thickness d. The breadth of the angle range at which the hologram becomes visible (is reconstructed), for example under monochromatic illumination, then depends solely on the layer thickness d. On illumination of the hologram with white light, for example, at a given illumination angle, the breadth of the spectral range which can contribute to the reconstruction of the hologram likewise depends solely on the layer thickness d. The smaller d is, the greater the respective breadths of acceptance. Therefore, if the intention is to produce bright and readily visible holograms, the aim is a high Δn and a low thickness d, so as to maximize DE. This means that, the higher the Δn, the more freedom is achieved to configure the layer thickness d for bright holograms without loss of DE. Therefore, the optimization of Δn is of major importance in the optimization of photopolymers (P. Hariharan, Optical Holography, 2nd Edition, Cambridge University Press, 1996).
WO 2010/0036013 discloses writing monomers based on (substituted) phenylcarbamoyloxyethyl propenonates and the use thereof as writing monomers in photopolymers for production of holographic media. It is possible to write holograms into these media with high diffraction efficiencies (DE).
However, the known holographic media do not have sufficiently high stability to varying humidity conditions for all applications. For instance, the optical function of exposed media changes considerably in some cases depending on the humidity that exists in each case. The effect of this is that the holographic media can reliably fulfil their optical function only within a tightly defined humidity range. Conversely, they lose their function entirely or at least partly when they are used in an environment in which the humidity level is outside the tight specified range.